Ignition systems, and other control systems used in combination with internal combustion (IC) engines, such as fuel injection systems, and the like, utilize sensors which provide output signals characterizing the respective instantaneous position of the crankshaft or of the camshaft, respectively, of the engine. Two types of sensors are customarily used. One type is the incremental sensor. It is usually either a star disk, or a gear, for example the gearing of the starter gear, in which a plurality of projections or gear teeth are uniformly distributed with intermediate uniform gaps. The number of the teeth is counted, starting from a predetermined reference tooth. Any angular position of the crankshaft or the camshaft, respectively, can thus be determined by counting the teeth from the reference tooth. Thus, an ignition instant can be calculated to provide proper ignition timing for the IC engine. To provide a reference, the incremental sensors may use separate reference markers. It has also been proposed to provide a gear, in which one of the teeth is formed in a shape which differs from that of the others, so that, in this particular manner, a reference tooth can be characterized, that reference tooth then forming a reference marker. Such an arrangement is described in the German Patent Disclosure Document DE-OS No. 29 39 643.
In operation, incremental sensors operate satisfactorily. Upon starting, however, defining a predetermined ignition instant is possible only after some dead time has elapsed, which will depend on the position of the marker tooth when the engine was previously shut down with respect to a fixed sensor. Thus, counting of the teeth, and hence providing predetermined ignition signals, can commence only after the reference marker, or marker tooth, has passed the sensing element. In dependence on the instantaneous position of the reference marker, more or less time will elapse after re-starting of an engine until proper operation of the incremental sensor is possible.
Another type of sensor is the segmental sensor, in which a transducer disk is provided which carries a plurality of elongated segmental portions at the circumference thereof. The number of the segmental portions corresponds to the number of the cylinders of the internal combustion engine when the segments are secured to the camshaft of the engine. When the segments, or the segmental disk, are secured to the crankshaft of the engine, then the number of segments corresponds to half the number of cylinders, since the crankshaft, as is well known, rotates at half the speed of the camshaft. The segments cooperate with a fixed transducer element which senses the leading and trailing flanks of the segments to thereby sense the length of the respective segments. The leading flank of the segments customarily is shifted with respect to the position corresponding to the top dead center (TDC) position of the pistons of the IC engine. The ignition timing, thus, can readily be defined within the timing period determined by the length of the segments. No specific reference marker is needed for the segmental sensors in order to determine the ignition timing, since the segments are associated with respective cylinders.
For some purposes, it is desirable to provide a signal representative of a particular reference cylinder. Thus, it is also desirable to provide a reference marker with segmental systems, and preferably, to secure the segmental disk on the camshaft.
It has been proposed to insert in the gap between segments an additional tooth or projection which is offset with respect to one of the segments and when the short or additional tooth passes the transducer element, in order to clearly define one of the segments, analyzing the signals derived from the short or additional tooth segments and the normal or long segments. Such a system is described, for example, in German Patent Disclosure Document DE-OS No. 28 54 017.
Use of a segmental sensor thus can provide a defined angular position of the camshaft. If such a segmental sensor is used in combination with a program controlled computer circuit which, for example, is a microcomputer controlling the ignition system of the IC engine combined with a motor vehicle, difficulties have been encountered since the additional leading and trailing flanks of the marker tooth will also affect the calculations of the program controlled computer. In a normal program controlled computer, the leading and trailing flanks are evaluated as asynchronous events, and applied to an interrupt input of a microcomputer. Occurence of any interrupt signal causes switch-over of the computer to enter a subroutine or a subprogram. An additional marker tooth, which will generate its own leading flank and trailing flank in the fixed sensor, can be sensed. In order to prevent the additional marker tooth, however, from affecting the normal program of the vehicle computer, substantial additional programming is necessary, which requires further memory address locations and substantial expansion of the memory capability of the computer. Thus, use of such an additional marker tooth is undesirable when an automotive-type computer is to be controlled thereby.